I haven’t seen this reported anywhere else, so I might as well throw a post up about it.
Valve recently launched their Steam Wallet feature, which was widely reported, but I just found a way to add custom amounts to your Steam Wallet (so you won’t be stuck with leftover pennies).
Based on this interview, this looks like something they intended rather than an exploit, so I feel comfortable using it:
As gamers who’ve used these kinds of systems before, we wanted to avoid some things we didn’t like. One was being forced to add more funds than we wanted (and seeing purchase prices that didn’t seem to match the funding options) and the other was being forced to do conversion into some virtual currency in our heads.
We’re not doing either of these in the Steam Wallet.