Maybe a silly question but are you looking for a spreadsheet app for a specific purpose or just general spreadsheetery stuff? In the case of the former you may find there's an app for the specific purpose. Just a thought.
Like Matt says, depends what you want from it really - a lot of the free ones are very basic. "Spreadsheet" by App.Authors Ltd is pretty good, costs about a fiver I think. Apple's "iWork" suite (Numbers, Pages and Keynote) is rumoured to be making it to the iPhone soon - if this happens then Numbers would be the one to have.
If you haven't discovered it yet, then Simplenote is a brilliant plain-text note taking app that synchronises 'over the air' with a variety of desktop apps, plus the simplenote website; I use it for all my job-related work notes and it's fantastic - and free
Thanks guys. It is for a couple of specific spreadsheets (simple ones) that i normally have on my pc, but it'd be really useful to have them on my iphone.
Have you tried using Google Docs through the browser? I have had some success with it on the iPhone. Plus it would allow access to the same document from both PC and Phone.