Do you mean online directory sites like
this one? Ours, and all the others I've seen like it, is done through
HEP Development Services. (I am not sure exactly what the name of the service is.)
HEP also publishes the
Matching Gift Register, which I find absolutely invaluable. I crack it open at least once a day to check a match policy or get contact information.
The online lookup and the paper register list a lot of the same information--such as match ratios and limits, and the procedure an employee must follow to apply for the match.
I believe the online lookup is mostly geared for donors to check whether their companies match gifts. But I refer to it fairly often myself, because it's the most up-to-date, and it contains the full notes for each company (these are frequently curtailed in the paper register). I also like the convenience of typing a name, rather than flipping through the main listings and possibly the indices. There are sometimes companies in the online lookup that aren't in the paper register (recent additions to HEP's database, I think).
On the other hand, the paper register really is my go-to resource, because it includes many useful details that the online lookup does not. Things like what kinds of organizations the company will match to (e.g., cultural organizations), when matching funds are disbursed, and mailing addresses for match programs (online lookup gives only phone and email).