Until a few months ago, only one thing came to mind when I heard the word:
Rolodex cards come in a few basic sizes: 2.25x4" seems to be the most commonly traded size, but I've also seen 3x5" and 4x6". Different sizes have their notches spaced differently, so different-sized cards require different holders. For this reason, many traders will start with only one holder and trade for that size. (I trade 2.25x4".)
Here are some examples of rolos I've made so far:
Other examples are in my Gallery. :-) You can also search for them on Google Image and Flickr.
Interested in giving altered rolos a shot? Come join us on Swap-Bot! In fact, I'm currently hosting a rolo swap that's open to newbies. *hint, hint*
'Til next time!