Apple - Are there any free and lightweight spreadsheet programs for the Mac?

If Chrome isn't too heavy for you Google Docs has very capable spreadsheet facilities that improve all the time. You can even run it offline if you're going to be away from an internet connection.

If you just need to view an Excel spreadsheet (.xls or .xlsx extension), you may be surprised to learn that Preview in Mountain Lion (and Lion, possibly?) can open and display them quite competently. You won't be able to edit anything, and I'm sure there may be problems with more advanced functions and macros and such, but if you just want to quickly view or print a spreadsheet, it may do the job.

Numbers, part of Apple's iWork suite, is neither free nor lightweight. But at US$20, it's quite affordable, particularly in proportion to the functionality it provides, and it certainly feels more lightweight than Open/LibreOffice.

Update 7 Feb 2018 (since this post seems to have got some attention recently for some reason): Numbers is now free in the Mac App Store and has been for some time. If Preview isn't doing the job or you do need to edit the spreadsheet, give it a try.

I haven't used it, but pyspread looks promising:

Pyspread is a non-traditional spreadsheet application that is based on and written in the programming language Python.

The goal of pyspread is to be the most pythonic spreadsheet.

Pyspread expects Python expressions in its grid cells, which makes a spreadsheet specific language obsolete. Each cell returns a Python object that can be accessed from other cells. These objects can represent anything including lists or matrices.