A list of various tools which can aid in the creation of new mods.
VTFEdit is an application that allows you view, and export Valve Texture Format (".vtf") files, import other image file types for viewing and export, in addition to editing Valve Material Type (".vmt") files.
GCFScape is an application designed to let you explore the different type of container files Valve uses for their games. Relating to Team Fortress 2, you can use GCFScape to open up the VPK files located in your "tf" folder. With access to the VPK's contents, you can extract the various files which you later wish to modify.
HLMV (Half-Life Model Viewer)
HLMV is the program used to preview 3D models created for Source. Quite frankly, it is useful for a great number of things including its ability to preview what your mod looks like without having to launch Team Fortress 2. HLMV is included in the game's files and its executable can be found by following, "Steam/SteamApps/common/Team Fortress 2/bin/hlmv.exe".
Image Editing Software
When making textures, your image editor of choice does matter to an extent. While most can help you reach the same goal, they all have different methods to getting there. It is important that which ever image editor you choose, you are comfortable using it and know how to make use of its features. Adobe Photoshop has a price tag, but you can find a free trial available on their site. GIMP on the other hand is completely free to use while maintaining many of the features you'd find in Photoshop.
VTF plug-ins are a valuable tool that is used in conjunction with your chosen image editing software. When installed, they allow you to directly import and export ".vtf" files from the image editing software. However, be aware that the plug-ins are not flawless by design. Take care in using them.
Audio Editing Software
Like image editing software, your audio program of choice can make a difference. If you aren't doing anything too elaborate, any such program should work just fine. The two we'd recommend is Adobe Audition and Audacity. Audacity is great because it is available for free while still being fully featured. Adobe Audition on the other hand cost money but provides a much more refined user experience. Both have extensive external resources available to help you learn how to use them.
3D Modeling Software
To create an item from scratch, you need 3d modeling software. Autodesk are creators of many comprehensive productivity applications including 3ds Max and Maya. While both tools may be used for creating 3d models, you'll find Maya has a stronger focus on animation. While they technically have fees associated with them, by signing up for an education account, you can download their software in full, for free. Blender is the completely free open-source option from the list. It packs all the features you can expect to need with a strong community for support. Finally it is worth checking out 3D Coat, an inexpensive user friendly 3d modeling program available through Steam from Pilgway.
When you create a mod, you will want a way to share it with the world. If you post to our forums, you can use the built in attachment function. However, collaboration becomes a big thing when working on custom content. Having an easy way to share that content at various stages of its creation is important. There are many options to choose from such as Dropbox, Google Drive, and MediaFire to name a few. Each service has it's own pros and cons. If you do your research, you are sure to find the service that best suits your needs.