Pine sol or simple green is the best "nature friendly" option. Brake-fluid, while effective is dangerous if you use it too much or aren't careful with it:
Here is the MSDS (Material Safety Data Sheet) on regular every-day brake fluid:
Acute Health Effects: From MSDS
Eye Contact: May cause severe irritation, temporary changes in the corneal epithelium, and diminished sensation.
Skin Contact: May cause irritation. Brake fluid may be slowly absorbed through the skin. Excessive exposure for extended periods of time involving large areas of skin would be necessary for absorption of harmful amounts.
Ingestion: Kidney damage and hepatotoxicity may occur if substantial amounts are ingested.
Inhalation: Inhalation is not expected due to the relatively low volatility of this product. Inhalation of fumes or mists caused by heating or agitating may cause respiratory irritation and have a systemic effect.
Chronic Health Effects: MSDS:
Repeated inhalation, ingestion or skin absorption of small amounts over time may result in toxicity symptoms and may adversely effect the kidneys. Chronic glycol ether inhalation has resulted in tremor, lethargy, headache, blurred vision, personality changes and coma.
YIKES!