There are still plenty of good, albeit old flash units around from the 35mm film days, i.e. the non-electronic age. E.g., I have an old 1980 Vivitar 283, and despite its age, it still works. However, one has to be rather careful using such flashes, particularly on digital cameras, since apparently some flash units have very high trigger voltages that could damage the electronics. I checked my Vivitar 283 using a small volt meter, and indeed it has a voltage of over 250V on the hot shoe trigger (one contact is on the bottom in the middle of the flash, the other contact is in the groove on the side where it slides into the hotshoe).

According to various info, Canon cameras can use 6V on the hot shoe and 250V on the PC connection (at least older digital cameras). However, most of Canon's lower range cameras only have a hot shoe, so a flash should have a 6 Volt limit.

Kevin Bjorke's web site (bozilla) has a long list of old flash units and their voltages. So, beware of using old flash units.

If you do want to continue using an old flash unit, there are various ways for remote controlling a flash, e.g., some listed here. Newer Canon cameras have built in wireless flash triggering, but again, you would need a receiver for your old flash. There are also hot shoe adapters with a special decoupling circuitry (most adapters just pass through the contacts - so beware), but they are rather expensive in relation to the limited functionality they offer.