5 letter .com names are an area that provide opportunity if you know what to look for in the name.
Catchy, memorable, easy to spell. A shorter name is worthwhile in a 140 character and mobile world. Of course even shorter could possibly be better, but you are going to pay a lot more for a two or three letter.com.
Its an area that even the new domainer can at least afford to dabble in.
This past Dn Journal had the following 5 letter .com sales:
yelli.com $2500 10/2005
askoo.com $2000 5/2006
hudou.com $2482 2007
You will see some of these names were regged not too long ago, and provide a nice return on a hand regged name.
Of course every 5 letter .com is not worth a lot, most are not worth the reg fee. But if you can find the right ones, with the right metrics you can make a very good ROI without investing a lot of money.
This is not a recommendation to go out and reg a bunch of 5 letter.coms, but rather to be on the look out when you see them drop, or check some handregs and maybe you will find something that meets your criteria.
Best of luck.