Ebay and other places are full of older manual lenses which are often every bit as good optically (and sometimes better) as their modern counterparts at a fraction of the cost. It is possible to mount these older lenses on most dSLRs with the appropriate adapter, and benefit from outstanding optics for a very small outlay indeed.

You will have to focus and set the aperture manually, but that's a small price to pay if you are getting results that might have cost you several hundreds or a thousand Pounds/Dollars/Euros more, for an equivalent lens in fully-auto mount.

I'll start with Canon, for that's what I know about the most. The Canon EOS system has a register (distance from lens mount to film plane) of 44mm. Many of the older lens systems had a longer register distance, so that immediately opens up the possibility of using a mechanical adapter to mate the two.

Some examples are:

1. M42 Screw-mount. Register distance 45.46mm.
 (Used on early Pentax, Praktica, various 60's Japanese cameras)

2. Pentax K-mount. Register distance 45.46mm
(The K-mount was also used by various Japanese cameras of the 70s onwards)

3. Contax / Yashica. Register distance 45.5mm

4. Olympus OM. Register distance 46mm

5. Nikon F. Register distance 46.5mm

6. Various Medium Format lenses.

These are just the most commonly available lenses you will find on ebay - there are many other third party lenses made by reputable lens makers, such as Tamron, Tokina, Vivitar, Sigma, etc. Most of them are perfectly ok for general use and some of them are outstanding.
For all of the above you will find adapters to allow you to fit them on a Canon EOS mount from sellers on ebay, mostly at very low prices. The quality of fit and finish of some of the older lenses is hard to match nowadays, unless you pay a serious amount of money. That's not to say they are all great - indeed, there are some utter dogs out there.
As ever, you have to be reasonably astute when you buy a lens, same as anything else - ascertain if the lens is clean, free of fungus, the aperture blades are not sticky, and the general overall condition is ok. Even if it looks great, it might still be a lemon, but so are many new lenses. There is a certain amount of gambling in buying an older lens, but the vast majority of the time it's a rewarding experience using a piece of glass that would have cost more than a week's wages 30 years ago and you've just got for a few quid.

Best of luck, and enjoy focusing manually.