As in texture packs? - They are allowed yes, as they just change the look of the game, not changing gameplay (except xray texture packs, which are considered cheating).
It's really down to personal preference, and what sort of look you're after - there are very "modern" looking packs, very cartoony ones, medieval ones......
I personally always use the default one. I've recently tried out the faithful pack and I quite liked it. If you like steampunk, there's the "Glimmar's" pack which is awesome.
Someone posted about tex packs a few weeks ago, which has a couple of links to try:
Texture packs suggestions (1.3.2)
The only thing to remember is that you need to make sure whichever pack you use is updated for whatever minecraft version you have - or else you won't see some of the newer blocks (because they won't have existed when the texture pack maker made their pack).