Yeah - the Steam Achievement Manager. I used it back then and didn't suffer any repercussions. The trick was to not unlock everything at once like a dork. ^^
But yes, I agree with Saint that you should always be wary of such tools - you are breaching the license agreement after all (third party program hooking into Steam). Nevertheless don't worry about permanent bans and such - the prog doesn't go anywhere near VAC secure servers. I seriously doubt Valve can even distinguish between this program's connections and real client connections without doing some tricky update to the client first.
P.S. The tool seems to have some problems now with servers saying that the client is out of date. Perhaps Valve already did something. Update: Never mind - it got updated. ^^
Your client sends all sorts of data to steam, more then the gameservers do.