I'm looking for a free spreadsheet editor that is simple and lightweight. I don't want to install OpenOffice just to use a some spreadsheets every once in a while.
If you just need to view an Excel spreadsheet (
.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.
There is gnumeric which is Gnomes's spreadsheet. It seems quick and lite. However it is designed for GNOME, one of the Unix desktops and does not look OSX like (note this is even using Cocoa GTK not X11)
Also you need to install on OSX from source. I used macports
sudo port install gnumeric which for Snow Leopard and Lion will install binaries. I suspect fink or homebrew will provide gnumeric as well.
I have used Tables, which I can recommend. The problem I found, however, is that successive versions of your sheet is not saved by Dropbox, because Tables saves its data in a proprietary format, as several files in a folder, and Dropbox will not save versions of folders, only files. Thus, I couldn't restore previous saved sheets from Dropbox. (I didn't check the behavior on Time Machine.) If this doesn't bother you, however, it's a speedy and useful program for small jobs.
the lack of a true mac native and lightweight spreadsheet has also bugged us for years, therefore we are developing a solution:
note that later-on advanced features like graphs will probably be unlock-able cheap in-app purchases. we aim to have standard functionality free.