You can use scribd to display spreadsheets in a wordpress.com site.
Excel can also output HTML, but it outputs an entire web page instead of just the table HTML, so you have to selectively pull just the table stuff out. The problem is though, the HTML formatting that Excel uses is mostly depreciated or flat out obsolete, so there is always a bunch of cleanup to do. Basically Excel and MS Word, don't speak web very well at all.
My suggestion for tables would be to use Windows Live Writer blog client, which produces very clean and web compliant code for tables and such and I believe you can actually copy and paste directly from Excel into it. It also has the advantage of allowing you to publish directly from WLW with no need to go through the WordPress editor.