There are currently four different flavors of standard def DirecTivos:
1. Series 1 DTivos consist of the Sony SAT-T60, Philips DSR6000, and Hughes GXCEBOT. These units are identical internally. The Hughes and Philips use the same remotes whereas the Sony has a unique remote and slightly different software interface but is functionally identical to the other two models. Sony stopped marketing Tivos after the series 1 models ceased production.
2. Philips DSR7000, Hughes HDVR2, RCA DVR39, and Samsung (?) series 2 models. These are all the same and are identified by the lack of an RID number on the back sticker. All series 2 models have USB ports on the back panel.
3. Philips DSR-704 and DSR-708, Hughes HDVR40, RCA DVR40, Samsung 4040, 4080, and 4120. These models are identified by the fact that they have an RID number on the back panel sticker.
4. DTV model R10. If you buy a new DTivo from DTV then this is probably what they'll send you.
All of the DVRs listed under Nos. 1 thru 3 above have all been hacked to allow functionality above and beyond what they can do right out of the box. The R10 has not yet been hacked via software but can be modified via a PROM mod that requires extensive technical expertise (i.e., don't try this at home).
The HR10-250 HD model will probably be replaced with a DTV (non-Tivo) model when they start switching over to mpeg4 transmissions later this year. DTV and Tivo have severed ties so any new models marketed by DTV will all be non-Tivo designs.
Sorry about the lack of info on the Samsung models as I can't remember the exact model numbers. They still fall into the categories indicated and can be identified as I described so ask the seller questions if you're looking to buy on ebay. Make sure any used DTivo you buy is guaranteed to work and that the buyer will refund your money or replace it with a working unit if it arrives DOA. I'd recommend using PayPal Buyer Credit for any such purchases as they will reimburse you if the seller sticks you with a lemon.