The best way to install texture packs is to use the MCpatcher utility which opens and modifies the jar files for you. It also backs up your original files automatically. It is also possible to use the classic technique of unzipping texture packs and then copying them into the Minecraft,jar file yoruself. See the texture pack instructions for details on installing particular packs.
Easily the best Minecraft texture pack (with extras, skins etc). This texture pack contains so much that we could spend all day listing the details. Just to give a little taster, this pack provides enough content for you to create epic sized constructions in the style of Victorian science fiction (steampunk). The textures are very detailed and professionally made. Even the mobs such as Zombie have glowing red eyes and a twisted face. Fantastic!!
Note: It's a 5MB download this one!
Dokus' RPG texture pack
One of the very first Minecraft texture packs and still one of the most reused and worked upon. Contains changes to all the fundemental elements with a touch of SNES about it. A great pack well worth downloading.
Note: It's a 2MB download.
Misa's 64x64 Realistic
Brings a dark medieval feel to Minecraft but with consistent and professional graphics. Misa's texture pack requires MCpatcher to install. This is the only way to install textures bigger than 16x16.
Warning: It's a 10MB download!