Although I have a couple of (loaned from friends) signal generators, these are rather large in a small shack. There are times when something small but calibrated is useful. Elecraft sell the XG3 signal generator which can be PC programmed to give a number of fixed output levels on a wide range of HF, VHF and UHF frequencies. It is the same (small) size as the T1 auto ATU i.e. about the same size as a packet of cards.
Norcal also sell a kit for a small fixed frequency signal generator called the
S9 Signal generator that can provide a fairly well controlled range of output levels. Currently it is out of stock, but the circuit is simple and could easily be built from scratch "dead bug" style.