FBI agents have been issued Glock .40 cal autos for about the last decade. Glock 22's were issued for many years to new agents, currently Glock 23's are issued. This has more to do with availability from Glock than with any preference: either model was available to agents changing over from SIGS or S&W 1076's for many years based on personal preference. Glock 17's and 19's are available to new agents who have demonstrated recoil sensitivity to a significant level.
The FBI mandated changeover to Glocks about a year and a half ago. The S&W 1076's were called up several years ago, as were the SIG 226's. Numerous issue SIG 228's are still being carried, however.
FBI SWAT has the option to carry a target-grade Springfield 1911.
Approved personal weapons have included revolvers, certain S&W autos, certain SIGS, and certain Glocks, most of which have been phased out in the past 10 years. Approved SIGS included the P220, P226, P228, & P225, as well as the P229 in .40 cal. Many of these are grandfathered in, as are Glock 22's, 23's, 26's, 27's, and 21's. Currently, only 26's, 27's, and 21's are eligible as new approved personal weapons.