In these period I've criticized a bit all the product launches that Google has done, because I think that after Gmail (undoubtely the best web-based email system) there was nothing really useful (maybe Google Calendar is an exception, because I think it's a good calendar system).
Today I've tryed the new Google Spreadsheets service (obviously, strictly on reserved beta stage, it's Google style! 
) and I'm really surprised: the system is really like a web-based Excel system, really useful if you're on travel and you need to create sheets or open Excel documents. I'd like if in the future also pivot functions will be added however... 
What I like is the "shared philosophy" around this product: you can invite other people to check and work on your document at the same time and chat with them too. How many times you've see the "the document is blocked by another user" message on Excel for example? Here the team work is a must!
Really a good product, now we're waiting the web-based clone of MS Word (how much will be the old Writely improved?). We'll see what happens