Depending on how good your domain name is I would also go for .info and also .org. I know that .org is a horrible looking tld but it can rank well on search engines and for the pennies a day extra it is silly not to lock it in.
For high profile domains I also lock in .tv .cc .biz and .bz
If your domain has a numeral in it, also consider registering the "spelled out" version, or vice versa.
Also consider registering "companynamesucks" types of domains -- might head off some potential public relations difficulties.
One of the largest traffic adult sites in the World today is <snip>
A big mistype traffic company, Netster have registered <edit> a misspelling </edit> and apparently get thousands of uniques PER DAY from mistypes looking for The <edit> site.
[edited by: NFFC at 8:38 pm (utc) on Aug. 23, 2002]
[edit reason] removed adult site [/edit]
.bz is the "country code" top level domain, i.e. cctld, of Belize. However, because of it's similarity to .biz those enterprising folks at enic (those of the bogus $1 million beauty.cc "sale") have begin to market it as an alternative "business" domain. It's nice and short and does look similar to .biz. Quite good if you did want a .biz domain and someone else bagged it first.
I personally hate the .biz domain. It always reminds me of some horrible MLM spam; "Go to www.MonsterWhoppingMLM.biz and make 3 billion dollars in 4 minutes, guaranteed!"