There's an interesting, if brief article in Wednesday's New York Times about "basic British bitters". They're increasingly served cask-style in fine establishments, but you can get them in standard tap or in bottles.
A familiar brand might be Fuller's ESB or the American Left Hand Brewery's Sawtooth Ale.
Anyway. Here's the article. They go on to review several other basic bitters. They're something to consider beyond your standard hef or witbier (or hop bomb!) in the warm summer months.