I've been given a long term loan of a 27" iMac. Looking at my photos on flickr and iphoto, they look, eh, just better, they seem to pop more than they did on the other PCs. Why is that? The other computers I am used to using are a dell desktop with LCD and low end dell laptop.
The 27" LED mac displays are "full gamut" displays, ones that cover around 98% of the Adobe RGB gamut. These are full 8 bit/channel (24bit) screens and offer a full 178° viewing angle. They are much higher quality displays than your average LCD screen, and specifically designed to output high quality, rich, saturated graphics. Additionally, Safari, which I assume you are using on the Mac, supports ICM. Browsers that support ICM will generally render photos with more accurate color than browsers that do not when an ICM profile is present in an image.
Your Dell computers are probably using much cheaper, 6 bit/channel (18bit) or 5/6/5bit (16bit) screens, and are likely not LED LCD screens but standard CCFL LCD screens. While these screens generally have higher refresh rates, and are great for gaming and movies, they do not reproduce color as accurately as diplays with higher bit depth and wider viewing angle. Contrast and color rendition on the cheaper Dell screens will generally cover the full sRGB gamut, but will fall quite short of the much richer and more saturated Adobe RGB gamut.
The Apple screens are middle ground, though. They use standard yellow phosphor/blue LED backlighting, which over the long term will often result in non-uniform color shift as the LED's age. CCFL backlit screens are less likely to encounter such color drift over the long term. For LED screens, full RGB LED will produce much cleaner, more consistent results over the long term. High end RGB LED screens designed for photo and graphics editing work will render the richest color (sometimes covering as much as 123% of the Adobe RGB gamut for maximum saturation) and purest white (blending RGB to form white more accurate than converting blue LED light into white light via a yellow phosphor, and is less subject to color drift over the long term.)
The 27" iMac has a pretty mid-high end LED backlit S-IPS LCD panel. Your low end Dell laptop and LCD monitors probably use TN panels which are inferior in color reproduction, angel of viewing, saturation, etc. Most TN panels are only 18 bit displays, so they will interpolate the 24 bit color your graphics card is putting out, while IPS panels will give you the full gamut, along with a larger viewing angle that has no color shift (something very common in TN displays).
In addition to what others said about the display quality, I would like to mention that there is no need to switch to iMac to see nice colors. Dell itself manufactures several display models widely considered as good for color-sensitive work. They also have IPS panels and wide gamut at a lower price than the Apple product. See for example
- DELL UltraSharp U2410 (24")
- DELL UltraSharp U2711 (27")
There are several reasons why you're pictures might look better. As a few answers have pointed out, iMac screen's are pretty decent for general color rendition.
Also, by default Mac's have a different gamma than Window's PCs. Mac's had a gamma of 1.8, and PC's have a gamma of 2.2--however since 10.6 snow leopard, Mac's now have a default gamma of 2.2.
Depending on the browser you use, you may or may not get color management. IE8 (the default windows browser) has no color management support; while Safari 5 (the default Mac OS X) does have color management. That means if your photos are tagged with ICC profile, the mac will likely render the color more accurately.
Save your photos with and without an embedded ICC profile. Try both sRGB and AdobeRGB. Open them up directly into your browser and compare them. You could also install a different browser or two on either system you use. Pretty sure there is a Windows version of Safari, and Firefox/Chrome are available for both. They all seem to handle color profiles different. Firefox is strange with color profiles. It is supposed to support them, but it always seems to oversaturate mine when I upload them with the ICC profile attached.