Spreadsheet
A few groups are working on spreadsheet activities for the XO. Dan Bricklin and Luke Closs are working on SocialCalc, one of the more active initiatives, which has a full javascript featureset and is pushing the envelope re: what js programs can do within Sugar.
See SocialCalc for the full activity description.
However, you can easily install "Gnumeric" which is an open source version of Excel. Enter Terminal activity. Type "su" and <enter> Type "yum install gnumeric" Follow the prompts.
When it finishes, just type "gnumeric" from the command prompt and it appears you then have a fully featured spreadsheet program! You may need to be a super user, by first typing "su" <enter>. (As a note, I'm using build 677 (I think), I've also installed "debian as an upgrade" as an alternate boot with XFCE).