EDIT: here's the link, which is different from what it used to be because I changed the folder structure of my dropbox:
http://dl.dropbox.com/u/15543016/lem/Lemmings%20Levels.xls-----
I have no idea if this has been done already, but I've been compiling all the levels into a spreadsheet for perusal. Source for Lemmings 2 and Holiday Lemmings was tle.vaarties.nl ; source for Genesis lemmings was half playing it through myself and half harvested from Planet Lemmings (or whichever one they get the screenshots from in the review thread).
The others are either harvested straight from the data files that come with the Mac version (which are essentially the same as the DOS version except for Tricky 21 and Taxing 22, where the goal is 100%) or come about as a result of me actually playing through the game. Lemmings Paintball is still a work in progress, however; I couldn't find a source on the internet with a list, so I decided to start playing through the game myself. I'm only up to Tricky 20-something (and the game's at my dad's because he has a windows pc, so I can't keep playing it).
It doesn't include Lemmings 3 because that game has no level titles, or indeed skills in the same sense. It doesn't include Revolution because I don't have access to it. I do have the PS2 version of Lemmings, so I should probably remember to play through the unique levels of that and add them. I don't have the PS3 version, however.
Anyway, I've highlighted quite a few of the cells, often automatically, basically where I think it's noteworthy or unusual. So, for instance, with number of Lemmings, 80 and 50 lemming levels are left with no highlighting but anything inbetween is given a colour, as they're unusual. Yellow cells in the skills columns are 0 - they're highlighted because I used to have each cell filled in with the numeral 0 and needed a way of visually distinguishing them easily, but for various reasons, mostly involving the calculated "Count" field which shows at a glance how many different skills are provided in each level, or how many levels each skill is provided in. For the Genesis version, I've highlighted new levels in blue, and changed levels in red.
I've added comments for the original/ONML levels, as well.
As for Lemmings 2, it's quite big, and only just fits on my rather large screen, but I've set the header and level info to be fixed, so that if you scroll, only the skills move. This should hopefully make it easier to read.
Also, if you try to sort it, be careful, because there are a few lines at the bottom calculating the average and so on, and these could be messed up if you moved them around, so select all the data before you do a sort on them.
Anyway, have a look through it; if you see any glaring errors, let me know!

Hope you enjoy. (If you can't view Excel files, let me know and i'll upload an exported version or something)
http://dl.dropbox.com/u/15543016/lem/Lemmings%20Levels.xls