I work in a hospital office setting. Attire consists of khakis and a collared shirt. I prefer low visibility, and around the 3" and under blade length. While I could carry a large knife, it stands out when I do have to use it. Personally I'd rather not draw any attention. My Spyderco Manix LW in CTS-XHP carries discreetly, but the time I used it in the lab the lab tech's eyes kind of bugged. Currently I rotate through a few different knives:
1) Spyderco Rhino - currently my favorite, though I wished it had a deep carry clip, or at least a black pocketclip.
2) Benchmade Bugout - the short deep carry clip is perfect. Great office edc.
3) Steel Will Mini-Intrigue - good knife for the money. Went with grey FRN as it blends better with khakis. Shiny pocketclip though.
4) Kershaw Dividend in M390 - I like this knife, but the auto-assist opening draws attention.
5) Spyderco Delica - I went with brown FRN to better blend with khakis. One of my favorite Spydercos.
Future considerations include:
1) Spyderco Chaparral FRN - not a big fan of choils, but I like the steel and it's a very discreet carry.
2) Spyderco Ikuchi - very cool knife and a slim package.
Of course there have been days I have gone old school and carried a slipjoint of some sort - Victorinox SAK, Case, GEC.