If you only need 50 mA SINK current, i.e. your "load" is connected to +5 and your requirement is to pull a relay coil, or bipolar transistor's base close enough to 0 volts to cause 50 mA to flow in that node, then you can do it with a darlington array such as the ULN 2803 as mentioned, or with a discrete NPN transistor, the open collector of which you can then use as your output. Keep in mind that the collector is an inverter, so it sinks current when the base is driven to a positive voltage, and it stops conducting when you pull it near its emitter voltage, in this case probably ground.

If you need only one driver and must both source and sink 50 mA, which is much less common, then you need an amplifier (OpAmp), of which there are many, the simplest of which would be a two-transistor complementary emitter follower which you can easily construct yourself. You'll have to read the datasheet(s) in that case.

RE