TransType from www.fontlab.com (US$179) or CrossFont from www.acutesystems.com (US$45). I don't have recent experience with either, but my guess is TransType might be the better one. Ideally, use OpenType fonts or your Mac freelancers may be able to use PC fonts (depending on app).
Both Crossfont and Transtype have strong supporters.
Do NOT convert between formats; convert Truetype to Truetype and Type
1 to Type 1. Other conversions are lossy. Opentype fonts are directly
usable on either Windows or Mac platform.
Note that some font licenses prohibit conversion, some allow
installation on multiple systems only in the same physical location.
There's an infinite number of combinations of license rules. Obtain
and read your (and your suppliers') font licenses.
Of course, there are shareware and freeware fonts out there, for which this treatment would be legal.
But, for retail font licenses, even if the license allows format conversion, I don't know of a single such license that would allow freelancers to give copies of the fonts they're working with to a client/employer. Certainly ours (Adobe's) does not, although in general it is one of the most liberal retail font licenses out there.
If a freelancer does work for you in Photoshop, do you expect them to give you a copy of Photoshop to continue that work? Of course not - you'd license your own copy. It's usually the same case with fonts.