Hey good topic! I've always been fascinated with guitar gear... though I haven't built any because I don't play... I'm just fascinated by gear that is designed to sound cool not nescessarily be transparent or blah, blah, blah. Anyway, I have some familiarity with the designs out there... so lets get thinking!
First thing to do is to set up your specifications:
A) How efficient is the speaker you are using?
B) How loud do you need it? (Take these two together to figure out the wattage you need)
C) Tubes/Solid State?
D) Is there any signature sound you are looking for?
E) How portable does it need to be?
F) How much do you want to spend
G) Any other things that it needs to have?
As minion said, the LM3875 or LM3886 is a great place to start for any amplification project. The common circuit that these are used in is called a GainClone (or some derrivation thereof) and is quite a nice performer. (These chips used to sit at the heart of VERY expensive Japanese equipment). If you need a moderate 50w or so they are a decent, cheap (maybe $50) way to go. They can also be bridged for more power if you like. The only disadvantages really are that they throw a fair amount of heat which can cause problems and unless they are bridged they do have power limitations.
If you need very low power... think about going class D. There are quite a few 10-25 watt class D kits out there for cheap. But you will need an efficient cabinet (hey that could be a fun project to!). Big advantage is that they generate next to no heat. You can even run them from batteries! Plus they sound pretty nice.
If you want tubes there are tons of designs out there... lots of schematics of vintage amps are floating around as well.
And if you want power there is always this:
http://sound.westhost.com/project117.htm