I found a nice thread at GardenWeb on long-blooming perennials. I definitely need to look at some of the plants mentioned. Here are some mentioned repeatedly:

• Savlia ‘May Night’. They say it goes for something like 18 weeks of bloom! Wow.
• ‘Walker’s Low’ Nepeta is also mentioned quite a bit.
• Geranium ‘Rozanne’ (a perennial, spreading, true geranium).
• Veronica ‘Icicle’ – also called Speedwell, it’s a vertical spiky flower.
• Kalimeris pinnatifida – a mum-like white daisy that flowers all summer.