As others have pointed out the Ikea Gallant system is superb for customizing different layouts at a reasonable price. It's got a steel frame under that tables for rigidity and load bearing... I could stand on it and jump up and down without fear of it breaking. It's also height adjustable. Get the T shaped legs over the individual legs for better looks and ease of adjustment.
Comes in 4 finishes, beech, birch, chocolate and white.
They make rolling drawer/filing cabinets to match.
IKEA | Desks | GALANT system | Accessories | Combinations
My last office in Vancouver was completely outfitted with Gallant and 3rd party hydraulic keyboard/mouse trays. It was great.