Can any telescope be used for solar observing? Can any telescope, such as a 8" reflector, that is normally used at night to look at planets be used or adapted for solar observing?
What kind of adapters or filters are required or is it better to get a dedicated solar telescope?
I'd like to look at sunspots, flares, prominences, eclipses, etc.
 A: To view the Sun in white light, you need a dedicated solar filter. Several types are available, but the best are made from AstroSolar film made by Baader. This is a polymer (not Mylar) with extremely good optical properties, coated with metal to reflect 99.99% of the light hitting it. It blocks most of beamthe Sun's light and heat before it even enters the telescope. Filters made from Baader material are available in high quality cells from Kendrick. I have several Kendrick filters and they are excellent:
http://www.kendrickastro.com/astro/solarfilters.html
To see flares and prominences, you need a much more specialized filter which only passes hydrogen alpha light. These can be used in front of refractors, but most observers prefer a dedicated solar telescope like the Coronado PST (Personal Solar Telescope). I use one of these every clear day to view the Sun. Small flares and prominences are visible nearly every day.
Although I have used projection to view the Sun, I generally recommend against it for safety reasons. It puts all the concentrated energy of the Sun into the focal plane of the telescope, and can very easily damage or destroy eyepieces. The projection beam leaves the eyepiece in a very concentrated form, hot enough to ignite paper placed in its path. I simply feel much safer with a professionally made filter blocking the light and heat before it enters the telescope.
A: There are solar filters that you can buy for most telescopes.  I have a 6" reflector and recently bought a solar filter for it and love it.
It's important to note that you really want a filter that will cover the full aperture of your scope (as compared to an eye piece filter) so that you don't focus the sun's intensity on to an eye piece.
A: What Water and Peter have mentioned is correct -- and the Balmer-alpha line (aka H-alpha) is the main spectral line that ground-based solar observatories observe, if they're not using a broad filter.
... but you're not going to see a lot of prominences, as they're more commonly associated with the transition region ... and for that, you're going to need EUV, which is blocked by the earth's atmosphere.
And for flares ... I don't know if you'd want to be looking at the sun during a flare, but although there are flares in the visible spectrum (and in radio, but then they call 'em 'radio bursts') whenever people talk about 'M class' and 'X class' flares, they're talking about X-ray flares, which again, is going to be blocked by the atmosphere.
Now, if you wanted to look for events from space-based telescopes, there's a number of resources for space weather observing, where you can get data and images from NOAA and NASA satellites in 'Near Real Time':


*

*SolarMonitor.org

*SpaceWeather.com

*Solar Dynamics Observatory

*STEREO beacon images

*TheSunToday.org's The Sun Now

*The Sun Today (at LMSAL)

*integrated Space Weather Analysis System

*Helioviewer
(disclaimer : I'm involved with a couple of those projects, but I'm in IT, not a solar physicist, so I likely need to get one of my co-workers to verify what I said at the top, once this site is out of private beta)
A: Yes, with a relatively inexpensive solar filter in front of the telescope almost any telescope can be used. The solar filter can be based on BoPET (trade name "Mylar").
To view flares and prominences a much more expensive (on the order of USD 1000 for a 8" reflector) hydrogen-alpha filter is needed.
A: If you want to project the image of the sun onto a screen, you don't even need (or want) a solar filter.  To do this you place a screen a good ways behind the eyepiece and focus the image onto the screen.  This works best if the image is not directly behind the telescope as it will be washed out by the direct sunlight.  
The exact distance depends on the size of the image desired and the size of the telescope.  Larger telescopes can make bigger images (i.e. have the screen farther away) as they collect more light.  We would do this all the time to show the sun to large groups of people at once (like during partial eclipses) with 8-12" telescopes.  I was always able to get a small image (about 4") with my 60mm refractor as a kid.
Warning - There is no filtering happening here so NEVER look through the telescope in this configuration.  Also there is a focal point between the eyepiece and the screen where the full power of the sunlight is concentrated so never put anything between there either or it will get burned.
BTW, you can do this with the full moon as well.  I have projected the full moon image from 8" telescopes as well so multiple people could view at once.
A: You may use mylar filters in front of the telescope, as other said. Another common way is to add a green glass filter at the entrance of the eyepiece (normally screwed in). This solution is normally shipped with the telescope, but I highly recommend against it. The reason is that the glass may crack due to the heat, which may compromise your retina if you are also observing when this happens. To reduce the issue, the telescope main entrance cover may have a lateral, independent, smaller hole, with a cap. It's specifically made for Sun observation, this reduces the amount of light entering the telescope, limiting the heat the filter have to take.
A: For viewing the moon I stopped down a 10" Newtonian reflector to 2", using tinfoil, thick paper, duct tape and a mason-jar ring (which is very round).  I think this might also work for a projecting an image of the sun but with a much smaller aperture. The tinfoil cover with over the aperture, with its hole hopefully will reflect away some of the heat. (from Central Texas where we will see only about 93%, looking forward to 2024!)
A: The technologies available for observing prominences are variable.  It's really not something to do until you have a good set of instructions and know what it is all about.  Most of the posts here are about white light solar filters which will show you sunspots and granulation on the sun's surface on a good day.  You will not see prominences.
H-alpha configurations for prominences come in various designs.  For some you do indeed put an Energy Rejection Filter or ERF on the front opening of the telescope.   But there are alternative designs called UV/IR blocking filters which go on the front of the diagonal (in a refractor).  The sun's infrared and ultra violet energies are reflected back out the scope.  It's really weird because you have a scope pointed at the sun and you think everything is going to get insanely hot, but everything stays cool to the touch.
To sum up, solar observing in white light with a white light filter is pretty easy to learn especially if you deal with an on-line provider for the white light filter.  You won't see prominences.  H-alpha filters such as the Daystar Quark sell for around $1200.  They require a DC power supply and various other things such as a UV/IR blocking filter or a front mounted ERF.  To venture into this domain it's a good idea to have a thorough background in the types of telescopes and follow instructions as to what will and won't work.  Coronado systems are very different from Daystar.  
Most of the time these days h-alpha is done in refractor telescopes in the amateur community.  White light is done across a wide variety of telescopes.  H-alpha viewing is at an expenditure level that most amateur astronomers consider to be high, so within the hobby it is a specialized activity.
