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HttpContext.Current.Cache.Items is an IDictionary, which means it would probably be safer to use IDictionary.Contains with the specified key. As is, your method will return false when the specified key does exist, but refers to a null reference (which should be valid) - that's a potential bug right there.

/// <summary>
/// Insert value into the cache using
/// appropriate name/value pairs.
/// </summary>
/// <typeparam name="T">Type of cached item (inferred from usage).</typeparam>
/// <param name="key">A string used to uniquely identify the added value.</param>
/// <param name="value">Item/value to be cached.</param>
public static void Add<T>(string key, T value)

/// <summary>
/// Insert value into the cache using
/// appropriate name/value pairs.
/// </summary>
/// <typeparam name="T">Type of cached item (inferred from usage).</typeparam>
/// <param name="key">A string used to uniquely identify the added value.</param>
/// <param name="value">Item/value to be cached.</param>
public static void Add<T>(string key, T value)

public static bool TryGet<T>(string key, out T value) 
{
    bool result;

    try
    {
        if (Exists(key))
        {
            value =  (T)HttpContext.Current.Cache[key];
            result = true;
        }
    }
    catch
    {
        value = default(T);
    }

    return result;
}

public static bool TryGet<T>(string key, out T value) 
{
    bool result;

    try
    {
        if (Exists(key))
        {
            value =  (T)HttpContext.Current.Cache[key];
            result = true;
        }
    }
    catch(InvalidCastException)
    {
        value = default(T);
    }

    return result;
}