After many years of being relegated to accent plantings, white-flowered plants are experiencing a resurgence of popularity. Using white not just as accent, but as a primary part of your gardening palate, can create a very strong impression.
In group plantings, white flowers create a pure, bright sea which can make any colored flowers you use absolutely jump out at you. White flowers also create amazing contrast with green foliage, especially very dark greens or even blacks and reds. White flowers are especially striking in spring. Try clematis or astilbe for bright white summer blooms. Hostas, though typically known for their foliage, often produce wonderfully bright silver or white blooms.