The following are two websites that I use frequently, and they have been extremely useful so I thought I would share them with you.
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The Hypertext D20 SRD
The SRD (or System Reference Document) is a website that contains all the open license information found in the Player’s Handbook, Dungeon Master’s Guide, Monster Manual, Epic Level Handbook, Deities and Demigods, and the Expanded Psionics Handbook (Plus the open content from Unearthed Arcana). The site allows for quick reference without the need for costly game books.
The website is laid out in a clear and organized manner so finding what you need is a breeze. The only drawbacks to the site (which are due to legal constraints, not design flaws) are that some content is not available. Anything that is property of Wizards of the Coast will not appear on the site. For example, Mind Flayer, Slaad, and Displacer Beast are copy written creations and cannot be published under the open gaming license.
The site itself is free, but it can be downloaded for greater ease and mobility for ten dollars.
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Monster Advancer ™
This site, true to its name, automatically advances the hit dice of the chosen monster. Also, the site has a number of templates that can be applied and then does the math for you. For example, you can select a basilisk, advance it up to 18 hit dice, and apply a fiendish template, click the button and a printer friendly version will appear in a new window. Along with the ability to increase hit dice, and add templates, the site allows you to add levels to your monsters.
I cannot begin to explain how useful this site is for busy DM’s that don’t have the time to sit and rework the monsters in their game. We all know how tedious it can be to add templates, increase the size, and add levels to monsters. The site breathes new life into monsters that your players have outgrown, and allows for maximum use of expensive miniatures. Haven’t been able to use that CR 7 Hill Giant because your party is too powerful? Literally, with two clicks from a drop down menu, you can advance that Hill Giant to 24 hit dice, add ten levels of Barbarian and you have a CR 20 bruiser ready to crush anything in its path.
My only complaint about the site (and it is a small one), is that the ability score increases that each monster receives every four levels are not automatically calculated. That said the advanced entry will have a line that tells the DM how many ability points the monster has accrued.
As with d20srd.org, the site does not have creatures that fall outside of the open game license. The site also has a quickened version of its program as well as a random monster chart generator.
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Both of these sites are free (though they do appreciate donations from grateful gamers), and they will make planning your games easier and quicker.
-Andy
Boccob's Blessed Blog 
