The poplin suit is a fairly popular summer wear option for men. It especially widespread in the Southern areas of the United States.
They are normally a cotton or cotton blend and a much lighter fabric than traditional wool. That makes them ideal for a hot, humid climate. You may occassionally find silk or polyester but its debatable as to whether that is true poplin. Poplin has a crisp feel but is generally a little less smooth than a twill.
One issue with the lighter fabric is that it will wear much faster than your wool.
There's always a little bit of a debate as to the proper times to wear poplin. Some people will say that its only summer (Memorial Day to Labor Day). Others say Easter to Labor Day. I say, wear it when it feels comfortable ie when its hot!
You can go with the traditional navy or dark colors with these, but other colors like khaki and olive can look great. This is summer wear and the lighter colors can go better with the sun and outdoorsy looks. You may want to consider a navy blue if you tend to wear them later in the fall season. Some people think navy poplin looks a little too much like a uniform. You'll have to try and see what works best for you.
Most big suit makers carry at least one style of poplin. Brooks Brothers in particular has a nice selection. Joseph A. Banks also has a huge selection of poplin for cheap, often under $200 a pop. Just keep in mind that these aren't meant to be your suit workhorse.