I'm not affiliated with any of these companies, and I don't make any money or anything if you use them. This is also my own personal experience, yours may vary.
ncix.com is the main computer store I use. On a part per part basis, you might find a cheaper price at other Canadian retailers (tigerdirect.ca, directcanada.com, newegg.ca), but overall they are the cheapest. They also have great customer support, good shipping rates and even do price matching. If you order a lot each year (say ~$3000/yr), their Premiere Partnership might save you some too. I've returned two things and haven't had any problems. Their website has a US version, but I don't know how the prices compare to US sites.
If you're in the market for a new monitor, you should first check out this detailed anandtech thread, and then head over to dell.com (or .ca, ...). When it comes to LCD monitors, its all about binning. You see, those Dell and Apple monitors might actually be made by LG and Samsung, but Dell and Apple get first pick at the best ones - so the LG made Dell monitor is actually better than the LG made LG monitor. Dell monitors aren't exactly cheap though...unless you wait for a massive sale like around Christmas. I picked up 2 27" high quality $1000 (CAD) monitors for $499/each - with free shipping. And their support is awesome. One of the monitors got a bright dead pixel and I had a new monitor at my door within 48 hours and easy (and free) shipping back for the broken one. Different monitors come with different warranties, so make sure to read the fine print.
Monoprice is actually the reason for this post. Its a great online store which I don't think enough people know about. Monoprice.com is where you goto for all your cable and electronic needs. It's dirt cheap. Shipping to Canada is a little on the expensive side, but even if it doubles your order, you'll still save a bundle. I've largely used them to order cables (CAT6, HDMI, Speaker wire, ...) but they have all types of neat stuff.
I'm a pretty big gamer, and despite owning a Wii and PS3, most of my video game purchases are done through Steam. Steam is a great service, but how you use it will determine what value you get from it. You see the trick is to buy single player games a year or so after their release when Steam puts them up for a huge 50% or more sale. At that point you can pick up last year's hit for $9.99 (or less). This past weekend I picked up Mirror's Edge for $4.99 and DeadSpace for $9.99. Multiplayer games are a bit different - you normally want to buy them when they come out so that you can be part of the thriving community - but even then, waiting a month or so might net you %25 off if you're lucky and you pay attention to the steam homepage.
I've only placed 1 order with culteryandmore.com so I'm not going to fully endorse them (before buying I did read a mix of reviews), but I was happy enough with the overall experience to add it to my list. We ended up buying a couple things, but the core of our order was a set of Wusthof knives (highly recommend!) Despite the shipping to Canada (which was reasonable) and duty, we ended up saving around $300. I've also noticed that they have great sales on their homepages, so its definitely worth keeping an eye on.
I'll group these up together since I've talked about most before. For a VPS I recommend Linode, cheap, powerful and flexible. For remote server backup, I recommend bqbackup.com. For unmanaged dedicated servers I recommend webnx or netdepot.