It’s hard to beat RuBisCO (Ribulose bisphosphate carboxylase/oxygenase) as an extremely abundant example of a protein that catalyzes several reactions (though at a single active site). Also, what about the proteins involved in developmental patterning that have different functions at different times in development? While a developmental protein may catalyze the same chemical reaction or bind to similar protein motifs at different times of development, these reactions may have very different effects on embryogenesis (see for example the pax genes). I think these examples are not exactly what you are looking for (as I understand it, you really seek genes with several distinct active sites having different activities), but I offer them to bring up other ways in which a protein can be multifunctional.

Bifunctional proteins, obtainable by genetic manipulation, composed of an interleukin-2 and a granulocyte macrophage colony stimulating factor constituent have the biological activity of both components but are distinguished by increased stability. These proteins are thus medicaments which are suitable for the treatment of malignant neoplasms.

Here’s some examples in my own field, the biology of the Endoplasmic Reticulum
i think the all time champion of such a protein from the point of view of doing two completely different things is Ira-1, the protein that initiates the unfolded protein response. this guy can phosphorylate other proteins but also has RNase activity. Also, BiP (or Kar2p) is a chaperone, but (if you believe Arthur Johnson) may also act as a plug for the Sec61 protein channel, and may also act to promote protein import into the ER. Hrd1 is a ubiquitin ligase that may (as is becoming increasingly clear) act as a protein channel for Endoplasmic reticulum associated degradation.

Please note: just because a protein can bring about two different outcomes doesn’t mean that it necessarily does 2 things. it may be that at different times is just activates different pathways by doing the same chemical reaction