Is there a program that allows me to not only read the contents of an Excel spreadsheet on a Mac without being a large download? I presume OpenOffice does it, but I don't have the bandwidth to download the full suite.
What version of Mac OS X - and what kind of extra features in Excel are you expecting it to support (eg. macros)
Mac OS X 10.5 and 10.6 provide the ability to view Word and Excel documents natively using either the QuickLook feature (just hit space in the Finder when you've selected the file) or TextEdit (in Word's case).
The other alternative I can recommend is to upload your .xls document to Google Docs - like QuickLook it's not perfect (no good for macros for example) but doesn't require any application to download.
If you require the use of macros and the full feature set available in Excel, your options are Microsoft Office, iWork (~500MB), or OpenOffice (171 MB)